The self-hosted OIDC dashboard provider was public-client + PKCE only, with
two `# TODO(confidential-client)` seams. Authentik and Keycloak commonly
default a new OIDC client to *confidential*, whose token endpoint rejects an
unauthenticated exchange (`invalid_client`) — so a self-hoster who accepts
their IDP's default could not complete dashboard login without manually
flipping the client to public.
Add optional confidential-client support:
- New optional `client_secret` (env `HERMES_DASHBOARD_OIDC_CLIENT_SECRET`,
or `dashboard.oauth.self_hosted.client_secret`; env-wins-config, empty
treated as unset). It is a credential, so docs steer operators to the
`.env` file; config.yaml is supported only for precedence symmetry.
- `_token_endpoint_auth()` selects `client_secret_basic` (HTTP Basic header)
vs `client_secret_post` (form body) from the IDP's advertised
`token_endpoint_auth_methods_supported`, defaulting to basic (the OIDC
default) when absent. Applied to complete_login, refresh_session, and
revoke_session (RFC 7009 §2.1).
- PKCE is sent in BOTH modes — the secret is client authentication layered
on top, never a replacement (OAuth 2.1 / RFC 9700 keep PKCE mandatory).
- Basic header url-encodes client_id/secret before base64 per RFC 6749
§2.3.1, so reserved chars (`:`, `@`, space) round-trip correctly.
Non-breaking: with no secret configured the provider is a pure public PKCE
client, byte-identical to prior behaviour (no Authorization header, no
client_secret in the body). The secret is never logged — register() reports
only a `confidential=<bool>` flag.
Tests: 16 new cases covering basic/post selection, default-when-absent,
public-unchanged contract, PKCE-preserved, reserved-char url-encoding,
blank-secret-is-public, refresh + revoke auth, no-secret-in-logs, and
env/config register wiring. Full dashboard-auth suite (nous provider,
middleware, gate, cookies, WS, 401-reauth, status endpoint) — 396 tests —
green, proving no existing auth path regressed.
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